Do you have a lot of target CATEGORY members? If it's not a long list you can:
PROMPT(COMBOBOX,%TCAT%,"Select Target CATEGORY",0,CAT1,{CAT1,CAT2,CAT3,CAT4,CAT5})
PROMPT(SELECTINPUT,%SELECTION%,,"Enter TIME selection","%TIME_DIM%")
...
TASK(/CPMB/CM_CONVERT,SELECTION,%SELECTION%DIMENSION:CATEGORY|ACTUAL|)
TASK(/CPMB/CM_CONVERT,TOSELECTION,%SELECTION%DIMENSION:CATEGORY|%TCAT%|)
...
TASK(/CPMB/CLEAR_SOURCE_CUBE,SELECTION,%SELECTION%DIMENSION:CATEGORY|%TCAT%|)
...
Please check other lines, there are errors in your script. I recommend to start with standard script and then modify it accurately!
Vadim